MEMs sensors and actuators
MEMs Sensors and Actuators
Micro-electro-mechanical Systems (MEMS) is a technology, which uses micro-fabrication techniques to make miniaturized mechanical and electro-mechanical devices and structures. MEMS devices satisfy the following criteria: they have a feature size of between one micron and hundreds of micrometer; they have some electrical functionality in their operation; they have some mechanical functionality and have a system-like functionality. These devices can vary from simple structures to complex electromechanical systems. The term MEMS is common in the US whereas the term they use in Europe is Micro Technology System in Europe and Micromachining in Japan (Patric Salomon and Henne van Heeren).
Mems Transducers Classification
The three major operations of MEMS are sensing, actuation and power generation, making microsensors and microactuators the most notable devices. Microsensors convert the mechanical input into electrical signals whereas microactuators use electrical signals to displace or rotate a mechanical structure (“About MEMS”). Microsensors and Microactuators are transducers, that is, devices that transform energy from one form to another. There are various MEMs transducers. In this paper the mechanical transducers, radiation transducers, thermal transducers and magnetic transducers are discussed.
1.0 Mechanical Transducer:
There are varieties of mechanical sensors. Classification is according to sensing mechanisms and parameters they sense (Ristic 173). So far; Piezoresistive sensors, Piezoelectric sensors, Capacitive sensors and Resonant sensors have been found to well with a variety of machines and machine needs.
Piezoresistive sensors utilize the Piezoresistive effect in which application of strain results in resistivity in the material.
Piezoelectric sensors use the piezoelectric effect where an applied force on a piezoelectric crystal results in a potential difference across the crystal.
Capacitive sensors rely on the basic parallel plate capacitor equation. Since capacitance is indirectly proportional to the distance between the plates, sensing of very small displacement is very accurate.
Resonant sensors consist of micromachined beams or bridges. They vibrate at their resonance when they are driven.
Here we have electrostatic and piezoelectric actuation types of mechanical actuators.
Electrostatic actuation: uses the principle of attraction between two opposite charged plates. They are simple to fabricate.
Piezoelectric actuation: In this category of actuation, the electrically induced strain is proportion to the potential difference. They have a disadvantage of being complex to fabricate and achieving small displacements.
There exist radiation sensors and optical actuators under this category.
Radiation sensors cover ionizing radiation, visible light, Infra-red and ultra-violet radiation.
Photodiodes: These are semiconductor devices for measuring light intensity based on the photoconductive effect. They are junction-based photoelectrodes with a p-n type of junction.
Charge-coupled devices (CCDs): are commonly used in handheld video recorders. When you apply a variation of control voltages to their surface electrodes, it results in the storage and transfer of photo-generated charge between elements.
Pyroelectric sensors: Illumination or temperature changes alter the charge to in the Pyroelectric sensors. Surveillance, military, and human motion detectors use this type of sensor.
Radiation (Optical) Actuators
The most common forms of optical actuators are the Light Emitting Diode and the Light Modulator.
Thermo-mechanical sensors: utilizes the fact that all materials have a coefficient of thermal expansion, therefore, when you sandwich two different materials together and undergo temperature change, movement in the sandwich assembly occurs.
Thermo resistive sensors:
Thermocouple: consist of a junction and measure the temperature dependent voltage that arises across the junction.
Incorporate tiny resistors that can locally heat specific areas when you control them.
They work because of the Hall Effect. They rely on the production of an electric field across a material through which an electric current is flowing and where a magnetic field is acting. The force applied to the charge carriers by the electric field exactly balances the Lorentz force from the magnetic field.
MEMs Fabrication Methods
There are three general classifications: surface micromachining, bulk micromachining, and high-aspect-ratio micromachining. MEMS fabrication uses a high volume of integrated circuit (IC) style batch processing that involves the additional or subtraction of 2D layers on a substrate based on photolithography and chemical etching.
Photolithography is the photographic technique to transfer copies of a master pattern, usually a circuit layout in IC applications, onto the surface of a substrate of some material (Lyshevski, 207).
Surface micromachining involves processing above the substrate, mainly using it as a
Foundation layer on which to build. Fusion bonding is a form of surface micromachining.
Surface micromachining involves processing above the substrate, mainly using it as a
Foundation layer on which to build. The two methods under bulk micromachining are dry and wet etching. Dry etching relies on vapor phase or plasma-based methods of etching using suitably
reactive gases or vapors usually at high temperatures while Wet etching describes the removal of material through the immersion of a material
Static and dynamic factors must be considered in selecting a suitable sensor to measure the desired physical parameter. Some of the typical factors are range, which is the difference between the maximum and minimum value of the sensed parameter. Accuracy, which is the difference between the measured value and the true value. Precision, which is the ability to reproduce repeatedly with a given accuracy. Sensitivity is the ratio of change in output to a unit change of the input. Resolution is the smallest change the sensor can differentiate. Zero offset that is a nonzero value output for no input. Linearity, the percentage of deviation from the best-fit linear calibration curve. Zero drift is the departure of output from zero value over a period of no input. Response time is the time lag between the input and output while bandwidth is the frequency at which the output magnitude drops by 3 Db. Operating temperature, the range in which the sensor performs as specified. Resonance, the frequency at which the output magnitude peak occurs. Dead band, the range of input for which there is no output and signal-to-noise ratio, ratio between the magnitudes of the signal and the noise at the output CITATION Abo15 l 1033 (About MEMS).
It is, however, difficult to find a sensor fitting the entire requirement, therefore, take a system level approach when choosing and not select in isolation
The solenoid is the most common electromagnetic actuator. A DC solenoid actuator consists of a soft iron core enclosed within a current carrying coil. When we energize the coil, there is establishing a magnetic field that provides the force to push or pull the iron core. We also encounter AC solenoid devices such as AC excitation relay CITATION Moh01 p 109 l 1033 (Gad-el-Hak 109). Normally, due to the spring force, the soft iron core is pushed to the extreme left position as shown. When the solenoid is excited, the soft iron core will move to the extreme right position thus providing the electromagnetic actuation. Another important type is the electromagnet. The electromagnets are used extensively in applications that require large forces CITATION Moh01 p 65 l 1033 (Gad-el-Hak 65).
There are many applications of MEMS transducers in different fields. The most common applications are in the fields of medicine and communications. In addition, MEMs transducers have found use in the fields of life science research and automotive engineering.
The MEMS pressure sensor some uses in the medical field. Monitoring of blood pressure in IV lines of patients in intensive care uses the disposable sensor. Also, they measure intrauterine pressure during birth. In addition, they monitor a patient’s vital signs, specifically the patient’s blood pressure and respiration in the hospital or ambulance. Moreover, drug infusion pumps of many types use the MEMS pressure sensors to monitor the flow rate and detect for obstructions and blockages that indicate that the patient is not properly receiving the drugs. The field of medicine has found tremendous use to the development of this kind of technology (Eloy 14).
High-frequency circuits are benefiting considerably from the advent of RF-MEMS technology CITATION Abo15 l 1033 (About MEMS). MEMS can improve electrical components such as inductors and tunable capacitors significantly when you compare to their integrated counterparts. With the integration of inductors and tunable capacitors, the performance of communication circuits will improve while the total circuit area, power consumption, and cost will reduce CITATION Adm15 p 45 l 1033 (Administrator 45). Also, various RF and microwave circuits need the mechanical switch that is a key component with huge potential. The samples of mechanical switches demonstrated have quality factors much higher than was previously available. Resonators as mechanical filters for communication are yet another successful application of RF-MEMS in circuits. The main driver for the introduction of MEMS microphones is undoubtedly its ability to withstand standard lead-free solder reflow profiles without affecting performance. Other advantages are you can integrate it with other electronics in one production process, reliability due to better vibration and temperature resistance and directionality when using arrays.
Life Science Research Application
MEMS is enabling new discoveries in science and engineering such as the Polymerase Chain Reaction (PCR) microsystems for DNA amplification and identification, biochips for detection of hazardous chemical and biological agents, and microsystems for high-throughput drug screening and selection. Polymerase Chain Reaction (PCR) bases newly developed systems on flow-through principle isolating nucleic acids and amplifying them
Different types of sensors are in use in the automotive industry to achieve different results. For example, the accelerometer that is a mechanical sensor, controls the antilock system in a vehicle, allows crash protection and skid control. Air conditioning in a vehicle uses the temperature, humidity, and light sensors while engine timing and transmission are because of the position sensor (Gileo 47). Over time there has been a great advancement and development in this field. A lot is still expected to be done over this MEMs technology
There is a link between the futures of MEMS to market trends in general. The increasing demand to monitor and control our environment and the equipment and instruments we use in our daily lives leads to the need for more sensors in cars, in industrial equipment and installation. Such sensors must be self-sustaining and able to communicate wirelessly for them to avoid the need for a multitude of wires. As a result, we need more sensors and small energy generating modules and wireless transmission components. The increased numbers of devices will clearly, drive size reduction, which in turn will enable higher levels of integration CITATION Abo15 l 1033 (About MEMS).
In the medical sector, the MEMS pressure sensor is the most common. However, there are new emerging trends such as the analysis blood chips that will continue to expand the MEMS technology (Marinis, 22). These trends are an opportunity for revolutionizing products in which we can integrate MEMS sensors.
Moreover, some of the most common devices that use MEMS technology such as a gyroscope, pressure sensors, and inkjet print jets require constant updating to keep up with increased technology. It, therefore, indicates that the research to improve these devices will ensure that MEMS technology remains relevant in the years to come (Kovacs 18).
Finally, the increase in national security requires small multi-parameter instruments that test air, blood and water for biological threats. The demand for such instruments is because of the increasing terrorist threats all over the world. MEMS devices will, therefore, be on the market as they meet the given criteria.
The MEMS technology is indeed the future. It will be possible when the industry removes the major challenges they face such as access to foundries, in design, simulation and modeling, packaging, testing and standardization. Despite these challenges, MEMS technology has come a long way and is here to stay. Scientists will still do more research to advance what already exist.
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